The observations recorded in the following pages were made in the course of three summers’ work at the marine laboratory of the Johns Hopkins University, organised and directed by Professor W. K. Brooks, and located for the past three years at Beaufort, N. C., where the material for this paper was collected. The abundance of Renilla reniformis (Cuv.), at Beaufort, suggested the desirability of a careful study of its embryology, and this was rendered still more apparent by the studies which Mr. Mitsukuri had made upon the anatomy of the adult organism.